The invention relates generally to the field of assemblies in which a substrate is mounted to a base carrier, and more particularly to the mounting of a metallized substrate having conductor paths on it to a base carrier having a substantially different temperature coefficient of expansion than the substrate.
In prior substrate mounting assemblies metallized ceramic substrates have been mounted to metal base carrier plates. The term "plate" as used herein refers to structure having top and bottom surfaces with a relatively thin dimension therebetween as compared to peripheral dimensions of the top and bottom surfaces. In some of these assemblies, semiconductor devices separately mounted and fixed with respect to a metal base carrier plate have been connected by wire bonds to bonding pads on the metallized substrate, while additional bonding pads on the substrate have had wires connecting these additional pads to lead frame projections in a housing fixed to the metal carrier plate. In such prior assemblies, typically the largest lateral dimension of the ceramic substrate was no more than one and a half inches. In such a situation, many different types of mounting techniques for attaching the substrate to the metal base carrier plate were utilized and provided satisfactory results since vibration and thermal expansion problems were not severe.
One of the above-noted prior mounting assembly techniques comprised providing a compliant adhesive urethane or acrylic adhesive tape over substantially the entire bottom surface of the substrate and between the substrate and carrier. Another of these prior substrate mounting techniques utilized a rigid adhesive to positively fix one edge of the substrate at which the bonding pads for the semiconductor die wires were to be attached while providing no support for the remaining portion of the substrate. However, neither of the above-described techniques are suitable for mounting metallized ceramic substrates having a substantially larger lateral dimension such as two inches or more. In such a case, fixing one edge of the substrate by a rigid adhesive while providing no support to the other end provides an unsatisfactory assembly which cannot pass mechanical vibration tests. Also, the providing of no support to a portion of the substrate hinders bonding wires to bonding pads at the unsupported end of the substrate. The use of a compliant adhesive over the entire substrate also does not provide a satisfactory bonding technique for large substrates. This is because movement, with respect to the base carrier, of the bonding pads at which the semiconductor die wires are bonded will induce mechanical fatigue in these wires and cause rupturing of these wires. This movement is caused by the thermal mismatch between the substrate and the metal base carrier. Applying a rigid adhesive over the entire bottom surface of the substrate also does not work since this will result in either cracks in the ceramic substrate or an adhesive bond failure due to the thermal expansion mismatch between the substrate and the carrier.